1. Field of the Invention
The present invention is directed to radio communications receivers which operate to detect radio signals contemporaneously transmitted by first and second transmitters and to recover first and second data, which the radio signals represent. Furthermore, the present invention is directed to a method of recovering first and second data from contemporaneously transmitted first and second radio signals. Yet more particularly, but not exclusively, the present invention is directed to wireless communications systems which include receivers arranged to recover data contemporaneously transmitted from a number of different transmitters.
2. Description of the Prior Art
Wireless communication systems are arranged to communicate data by transmitting radio signals, which are representative of the data, between transmitters and receivers of a communication system using part of the radio frequency spectrum. As a result of an increasing demand for radio communications, wireless communication systems are arranged to make optimum use of an allocated part of the radio frequency spectrum. To this end, wireless communication systems are arranged to operate in accordance with a multiple access scheme in which a number of data communications channels may be contemporaneously supported in a way which, as far as possible, makes optimum use of the allocated radio frequency spectrum. An example of such a multiple access scheme is time division multiple access in which data from a number of transmitters is communicated in a corresponding number of time slots into which a radio frequency carrier signal is divided. In this way a number of data communications channels are effected, with each time slot representing contemporaneously supported channel.
To further improve a utilization of the radio frequency spectrum allocated to a wireless communication system, receivers, which are arranged to operate within such systems, are known to include adaptive antenna arrays which utilize adaptive beam forming techniques to the effect of exploiting spatial diversity, which is inherent in the communication of radio signals.
An example of a receiver provided with an adaptive antenna array which exploits the spatial diversity is described in an article entitled "A Two Step Spatio-temporal Filtering Scheme for Smart Antennas Compared to a Multi-user Maximum Likelihood Algorithm", by C. Brunner, M. Haardt, C. Farsakh and J. A. Nossek, published in the Proceedings of 48.sup.th IEEE Vehicular Technology Conference (VTC 1998), Ottawa, Canada, May 1998. This known receiver is provided with an antenna array, which, in combination with a spatio-temporal filter, operates to separate radio signals contemporaneously transmitted from different transmitters to the effect that data represented by each of the radio signals may be recovered by a data detector to which the separated radio signals are fed. As such, a substantial increase in a number of communications channels provided by a wireless communications system may be effected by allowing contemporaneous transmission of radio signals. With this known arrangement, a data detector is assigned to each of the radio signals associated with a particular transmitter, wherein the radio signals have been separated in accordance with an angle of arrival of a wave front of the radio signals at the antenna array by the spatio-temporal filter, such that the data detector recovers the data associated with a particular transmitter.
The term wave front is used herein to refer to a component of a radio signal which arrives at an antenna array at a particular angle having reached the antenna array via one of a number of distinct propagation paths. Thus the use of the term wave front is analogous to its use in connection with the propagation of silent or compression waves, the front or leading part of which are known as "wave fronts".
A disadvantage with this known apparatus is that interfering wave fronts can only be canceled completely by the spatial filter if a relative difference in the angles of arrival of the wave fronts is substantially small. Furthermore, if, for example, a wave front of interest experiences a deep fade as a result of fast fading caused by the radio communications channel, the wave fronts reconstructed by the spatio-temporal filter may be corrupted by a stronger interfering radio signal wave front generated by another transmitter to the effect that a data detector cannot recover the data which the radio signals represent.